Over recent decades many new composite materials have been developed, some with very valuable properties. Advanced composite materials are widely utilized in industry due to their high strength, low weight, high degree of stiffness, and stability against dimensional variation. By carefully choosing the reinforcement, the matrix, and the manufacturing process that brings them together, engineers can tailor the properties to meet specific requirements, such as resistance to heat, chemicals, and weathering by choosing an appropriate matrix material. The greatest advantage of composite materials is strength and stiffness combined with lightness. By choosing an appropriate combination of reinforcement and matrix material, manufacturers can produce properties that exactly fit the requirements for a particular structure for a particular purpose. The right composites also stand up well to heat and corrosion. This makes them ideal for use in products that are exposed to extreme environments such as boats, chemical-handling equipment, and alternative energy mechanisms such as wind turbines. In general, composite materials are very durable. Such composites also have a high level of chemical and thermal stability.
There is often a need to connect composite parts, preferably using an adhesive to provide the highest strength joint. One of the difficulties, however, is joining these composites together to form variously shaped structures. In particular, wind turbine manufacturers applications demand adhesives that can withstand the centrifugal forces applied to each blade. They must at the same time bond very large components, such as the two halves of a blade envelope and the spar, while filling substantial gaps that are inevitable with joint structures up to 60 meters long. They must then maintain bond strength for the blade's lifetime under constant thermal cycling and environmental attack. Bonds must endure outside ambient conditions with daily cycling, without yielding for up to 25 years.
One of the methods employed in joining composite parts is application of a paste adhesive. Paste adhesives can provide high strength joints, but are difficult to process due to their high viscosity. Epoxy adhesives can also provide high strength joints, but processing of epoxy adhesives can be difficult. Epoxy adhesives are made by combining an epoxy resin with a catalyst and begin to react on admixture. The reaction changes the viscosity, making processing difficult. Also, cure time can vary dramatically with temperature. Some epoxy systems cure so rapidly at temperatures above 90° F. that they become almost unusable. At colder temperatures, e.g., below 60° F., some systems may take days or more to cure. Other potential adhesive materials like polyesters or vinyl esters do not provide sufficient adhesive strength.
Not only must the adhesive provide durable bonding, it must also be easily processed so that it can be used in a variety of conditions in the field. Adhesives that are commercially available do not meet all the requirements of adhesive strength, working time, and processability necessary for the bonding of composite parts of wind turbines, for example.
Accordingly, there exists a need to durably attach composite parts to one another.